Recently, a longer recording time, and higher recording density are strongly required of magnetic recording media such as a magnetic tape. In order to achieve higher recording density, a thinner and longer magnetic recording tape is required. A thinner base film, which is indispensable for a thinner tape, inturn results in reduction of stiffness of the tape; the tape edges are readily damaged in the course of loading or unloading, or when a momentary high tensive force is exerted on the tape, it sometimes is deformed, resulting in distortion in recorded data.
One conventional base film for a magnetic recording tape is a biaxial stretching polyethylene terephthalate film, in particular, a so-called "super tensilized" film intended for longer time recording, and is provided with a higher Young's modulus in a traveling direction. The maximum possible Young's modulus of a polyethylene terephthalate film in a transverse direction is approximately 800 kg/mm.sup.2. A higher Young's modulus in the traveling direction inevitably results in a smaller Young's modulus in a lateral direction, and, accordingly, the edges of such a recording tape during traveling are more prone to be damaged. On the other hand, increasing a Young's modulus in the lateral direction in a film manufacturing process inevitably results in an insufficient Young's modulus in the traveling direction, and, resultantly, poor contact between the film and a magnetic head causes fluctuation in output level.
A base film endowed with a higher Young's modulus by high degree of stretching is disadvantages in that distortion caused in a film forming process remains and results in poor dimension stability. High degree of stretching is further disadvantageous because of lower product yield.
This type of base film essentially requires the processes of thermal relaxation and/or aging to eliminate residual distortion, which makes it more complicated and difficult to select and combine the production conditions of the tapes.
Therefore, there has been limitation on the stiffness of conventional magnetic recording media, and, as mentioned above, thinner recording media lack in durability, resistance to damage, and preservability of recorded data. In essence, such recording media do not satisfy every requirement for recording media associated with a longer time recording operation.
Recently, as a result of more common use of a built-in camera type VTR, the cassette tapes are often subjected to the severe conditions outdoors and in cars. Accordingly, the tapes increasingly require dimension stability that prevents distortion, and, this in turn requires dimension stability of a base film.
Meantime, as a density and S/N ratio of magnetic recording media especially for VTR and computer are getting higher recently, magnetic powder of a smaller particle size is more commonly used.
Generally, S/N ratio of a magnetic recording medium is deemed to be proportional to the square root of a number of the magnetic particles in a recording material that is responsible for recording and reproducing data, and, accordingly, a particle size of a magnetic powder used is smaller, the S/N ratio is better.
Use of finer magnetic particles of a bigger BET value (specific surface area determined by Brunauer-Emmett and Teller's equation) results in smoother surface of a magnetic layer and smaller spacing loss. This fact is advantageous in attaining higher electromagnetic conversion properties.
On the other hand, too smooth a surface of a magnetic layer intended for higher recording density deteriorates the traveling properties of a magnetic tape, which has been the problem to be solved. Such a recording medium strongly rubs a magnetic head during recording and reproducing operation, and an increase of contact area and friction factor in turn causes deteriorated traveling properties. Such case as mentioned above causes further unfavorable problems such as abrasion of a magnetic layer in repeated use, falling off of magnetic particles, and clogging on a magnetic head with the particles fallen off. A thinner magnetic tape as well as a super-smooth magnetic layer causes tape sticking to a tape guide while the tape is forwarded or wound. Thus, the traveling properties of a magnetic tape can be readily deteriorated. These problems result in a shorter service life of a magnetic tape and adverse effects on the electromagnetic conversion properties such as S/N ratio and RF output.
These problems are increasingly notable as the result of recent development of higher quality, longer recording time type magnetic tapes. Accordingly, there have been strong demands for technological breakthrough on the dilemmatic problems of higher density magnetic recording media and thinner magnetic tapes versus higher traveling properties and durability of magnetic tapes.